The Mercenary Que option. It has been mentioned in the past... and ignored by SE.

It's probably ignored because it would have the same effect as eliminating GC's entirely. Consider if they implemented it. You have the Seize direct que (in which you que for your specific GC and are slotted accordingly) and then you have the Merc que (which is kind of like roulette. The DF would slot you into whatever GC needs the players). Which do you think would que faster? Merc. Every time. No one would que for their GC's anymore (except maybe already established pre-mades), which would make the GC que option irrelevant.

Another pitfall is the que with friends option. If merc ques were the solution to filling in the gaps of less populated GC's (in a perfect world, maybe) than what do you think would happen if people started quing with friends within the Merc option? The DF would have a heart attack trying to slot groups of 2-8 into empty GC spots that are only 1's and 2's. This problem already happens with pre-made groups. Sometimes it's faster (though less fun) to que solo for Seize. DF has an easier time finding spots for 1 player than it does for a full team of 8, and that's within the SAME GC. Imagine if DF tried to slot a pre-made merc group into a single GC when the empty spots are spread across all 3.

Don't get me wrong, I like the Merc idea. I always have, but for DF it's a matter of filling spaces as they come up, and Merc ques could be incompatible with the current GC set up. Merc ques would work fine if GC's didn't exist. They might even work fine with GC's if it's only used for solo ques, but pre-made merc teams would probably find themselves in que limbo while the DF tries to find an open GC spot that can accommodate their total party. The solution to this would be that everyone just ques for Merc... but, then that leads back to point one. Why keep the Gc's at all if everyone is queing for merc?

It's all about the logistics of how people are slotted in ques. SE's probably considering this right now, but the hard truth is that it probably just doesn't work while GC's are still in place.